Magnetic tunneling junction (MTJ) is a key component of both magnetic recording heads and magnetic random access memory (MRAM). A typical MTJ structure for a recording head or for an MRAM application is schematically illustrated in FIG. 1, as follows:
Buffer layer 11, antiferromagnetic layer (AFM) 12, an outer pinned layer, Ru, (neither shown) inner pinned layer 13 (the reference layer), tunnel barrier layer 14, free layer 15, and capping layer 16. In this structure, free layer 15 serves as the sensing layer which responds to external fields (specifically those stored in the media) while inner pinned layer 13 remains fixed and serves as reference layer. The electrical resistance through barrier (i.e. insulating) layer 14 varies with the relative orientation of the free layer moment relative to the reference layer moment thereby converting magnetic information stored in the media into electrical signals.
For effective operation as part of a magnetic recoding head, the basic requirements for TMR sensors are as following:
1) Low resistance·area product (RA)
2) High magneto resistance (MR) ratio
3) A soft free layer having low magnetostriction
4) Low interlayer coupling through the barrier layer.
5) Strongly pinned reference layer.
MgO-based MTJs are promising candidates for achieving high recording density and/or high frequency application because their TMR ratio is significantly higher than those of AlOx or TiOx based MTJs. S. Yuasa et al [1] and S. S. Parkin et al. [2] demonstrated that MR ratios around 200% can be achieved at room temperature in epitaxial Fe(001)/MgO(001)/Fe(001) and with polycrystalline FeCo/MgO/(Fe70Co30)80B20 MTJs.
Yuasa et al. [3] have also reported that TMR ratios as high as 410% at room temperature can be achieved in fully epitaxial Fe(001)/Co(001)/MgO(001)/Co structures. Meanwhile, D. D. Djayaprawira et al [4] showed that MTJs of CoFeB/MgO(001)/CoFeB structure made by conventional sputtering can also have a very high MR ratio (230%) with the added advantage of greater feasibility and uniformity. For low RA applications, the MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% at RA=2.4 ohm·μm2 was achieved by K. Tsunekawa et al [5] by inserting a DC-sputtered metallic Mg layer in between the bottom CoFeB and the rf-sputtered MgO, an idea initially proposed by T. Linn et al. [6] to prevent oxidation of the bottom electrode (CoFe) in a CoFe/MgO/reactive sputtering/NiFe structure. Also, Ta getter-presputtering prior to the rf-sputtered MgO layer can achieve 55% TMR with an RA of 0.4 ohm·micron2 as recently reported by Y. Nagamine et al. [7]. An alternative way to form a low RA MgO barrier is to deposit two metallic Mg layers with a natural oxidation process in between as we previously proposed for the benefit of better process control and MRR uniformity.
CoFeB material has been used in MgO based MTJs to achieve a magnetically soft free layer having a high MR ratio. High MR ratio and low RA has been demonstrated in MgO MTJs with CoFeB free layer. It was also demonstrated that insertion of a thin CoFe layer between MgO barrier and CoFeB facilitates getting a high MR ratio even at low annealing temperatures (ca. 300° C.). However, there remains a concern that a CoFeB free layer will have a high positive magnetostriction coefficient (lambda).
There are several possible ways to reduce lambda in a CoFeB based free layer. As shown elsewhere, lambda can be reduced by replacing CoFeB with CoB or by adjusting the CoFeB composition. However, magnetic softness deteriorated at the high annealing temperature needed to achieve a high MR ratio. To tackle this issue, a special annealing procedure was developed whereby a relatively high dR/R could be achieved while the free layer was still soft. An alternative approach for reducing lambda is to add a NiFe layer, which has negative lambda and is magnetically soft, on top of the CoFeB in the free layer. However, CoFeB/NiFe-type free layer structure is not usable because direct contact between CoFeB and NiFe causes a drastic drop in MR ratio.
H. Wang et al have proposed to use CoFe/CoFeB/Ta/NiFe as a free layer with high dR/R. In this structure, high dR/R can be achieved because the CoFeB is separated from the NiFe by a Ta insertion layer. The CoFe\CoFeB and NiFe layers are magnetically coupled through orange-peel type coupling, which tend to align magnetic moments to be parallel. However, this coupling is relatively weak and, in the case of real devices, has to compete with magnetostatic coupling from the edge of two layers which tend to align these two layers anti-parallel. As a result, magnetic noise for this kind of structure is relatively high. So although signal amplitude is high, improvement in signal-to-noise ratio is limited.